Contributions of femoral fixation methods to the stiffness of anterior cruciate ligament replacements at implantation.

One purpose of this study was to determine the stiffness of three femoral fixation methods used commonly in anterior cruciate ligament (ACL) reconstruction to secure a double-looped semitendinosus and gracilis (DLSTG) graft and then assess how the stiffness of these methods affects the stiffness of the young human femur-fixation method-graft complex at the time of reconstruction. A second purpose was to define principles for adjusting the stiffness of the ACL replacement (defined as the femoral fixation method plus DLSTG graft plus tibial fixation method) to match that of the native ACL. The stiffness of a DLSTG graft and the stiffness of the femur-fixation method-DLSTG graft complex for three endoscopic fixation methods were measured. Fixations of the DLSTG graft to a button, anchor, and post, both with and without compaction of bone, were tested in young, human femur. The stiffness of each fixation was calculated by modeling the DLSTG graft and fixation method as a series of springs. The stiffness of the DLSTG graft averaged 954+/-292 N/mm. The stiffness of the DLSTG graft-fixation complex was lowered fourfold to 40-fold by adding fixation. The method of fixation determined the reduction in stiffness. The stiffness of the femur-button-DLSTG graft complex averaged 23+/-2 N/mm, the femur-anchor-DLSTG graft complex averaged 25+/-3 N/mm, and the femur-post with bone graft-DLSTG graft complex averaged 225+/-23 N/mm (P = .0001). The knot in the suture loop was the least stiff component and determined the stiffness when the DLSTG graft was fixed with both the button and anchor. Compaction of bone significantly increased stiffness by an average of 41+/-14 N/mm (P = .027). Because the stiffness of femoral fixation methods are 4 to 40 times less than the stiffness of the graft, increasing the stiffness of an ACL replacement would be best achieved by selecting fixation methods with higher stiffness and not by either shortening the graft or increasing the cross-sectional area of the graft.